This invention relates to ballast apparatus for operating HID sodium lamps and, more particularly, to a lead-type ballast which programs and controls the operation of the HID sodium lamp.
Inductive ballasting devices for high-intensity-discharge (HID) devices are highly developed and are used to ballast high-pressure mercury lamps, high-pressure metal-halide lamps and high-pressure sodium lamps. The simplest of these ballast devices is a single reactor which can be used to ballast mercury lamps, providing the line voltage exceeds the minimum starting voltage required by the particular lamp. If the starting voltage exceeds the line voltage, the ballast can be formed as a high-reactance transformer which combines the transformer function and the ballasting function in the same core and coil assembly. For operation of mercury lamps, improved performance can be obtained by including a capacitor between the inductive reactance and the lamp. If the inductive component takes the form of an auto-transformer, these are referred to as constant wattage auto-transformer ballasts. If the lamp circuit is isolated from the line by means of a separate transformer, the ballast is known as a constant wattage type ballast. Both of these modifications which include a capacitor between the transformer and the lamp can be classed as lead-type ballasts. Such lead-type ballasts for mercury lamps have been made dimmable by incorporating a variable voltage source between the capacitor and the lamp. Thus, the voltage delivered to an operating lamp can be varied. Such a construction is disclosed in U.S. Pat. No. 3,015,058 dated Dec. 26, 1961. Another method for dimming such lamps is to utilize a plurality of parallel-connected capacitors between the transformer secondary and the lamp. By connecting and disconnecting the capacitors, the power delivered to an operating lamp can be varied.
Metal halide lamps are quite similar to the mercury lamp except that a higher voltage is needed to start the lamp and a higher reignition voltage must be present each half cycle. These requirements have led to the development of so-called lead-peak ballasts which utilize either a high reactance auto-transformer or an isolated transformer, with a capacitor connected in circuit between the transformer output and the lamp. U.S. Pat. No. 3,599,037 dated Aug. 10, 1971 discloses including a variable inductance between the capacitor and the lamp in order the control the current to the lamp. The variable inductance is controlled by a feedback signal which in turn is controlled by the light output of the operating lamp.
High pressure sodium lamp ballasts are of various designs and the most commonly used is the lead-peak type which includes a capacitor intermediate the transformer output and the lamp. Particularly in the case of high pressure sodium lamps, the lamp operating voltage tends to rise with burning time, with resulting increase in power consumption. This is the normal cause of failure for such lamps as determined by the inability of the ballast apparatus to operate a lamp at considerably higher-than-design voltage.
In recent years there have been developed a wide variety of control devices for sensing at least one HID lamp operating parameter and converting this sensed parameter into a signal which in turn is used to control a predetermined lamp operating parameter. One example of such a control device is described in U.S. Pat. No. 3,590,316, dated June 29, 1971. The circuit described therein senses lamp operating voltage and lamp operating current and converts this into a control signal which is representative of the actual lamp operating wattage. The control signal actuates control circuitry which maintains the operating lamp wattage at about a predetermined value. In another system, as described in U.S. Pat. No. 4,162,429, dated July 24, 1979, line voltage and operating lamp voltage are both sensed and converted into a signal which controls lamp wattage. In recent years, modified ballasts have been marketed which use a variable transconductance solid-state multiplier to measure the wattage input to an operating lamp. This is converted to a control signal which in turn operates to control the actual lamp wattage in a predetermined fashion. The basic transconductance multiplier circuit is described in U.S. Pat. No. 3,689,752, dated Sept. 5, 1972.
Another type of programming and control is described in U.S. Pat. No. 4,172,962, dated Apr. 3, 1979 wherein continuous time of lamp operation is sensed, and after a predetermined time has passed, the control circuitry automatically dims the operating lamp so that less power is consumed during the later portions of the nighttime when less illumination is needed.
U.S. Pat. No. 3,344,311, dated Sept. 26, 1967 discloses sensing a wide variety of lamp operating parameters in order to generate control signals which are used to control a lamp operating parameter. The embodiments disclosed include (1) sensing lamp current to control power input to the lamp, (2) sensing reactor voltage to generate a control signal which controls the lamp, (3) measuring light output from the lamp to generate a control signal which in turn controls the output of the lamp, (4) voltage across the lamp is measured and converted to a control signal which controls the output of the lamp, (5) lamp current is measured to control the lamp output, (6) both line current and lamp voltage are sensed and integrated to control lamp output, and (7) sensing input voltage variations which in turn are used to control the lamp operation.
In summary, the present state of the art has developed devices which sense a wide variety of lamp operating parameters in order to generate signals which in turn control a predetermined lamp operating parameter as desired for the operating lamp. The more advanced of these devices have their logic circuits incorporated into integrated circuit (IC) chips.
Add-on devices for modifying the performance of HID lamps on existing ballasts are known and in U.S. Pat. No. 3,925,705 dated Dec. 9, 1975 is disclosed an add-on device for lag-type ballast for reducing the wattage at which the lamp will operate.